Cleaning up the api-doc of ad_html_text_convert, after searching half an hour for something that would format the contents of a richtext widget and only finding this proc after looking into the display code of the form builder. (rant: It'd be awesome if those who add cool functionality would find the extra minute to at least mention it in the api-doc, /endofrant). Also changed it so that it returns the empty string when input is empty, so it can deal with NULL content/format pairs in the database conveniently.
